1. Field of the Invention
This invention relates generally to automatic controls for boilers, and more particularly to a microprocessor-based sequencer and method for operating the same, capable of monitoring changes in load demand and adjusting the firing rate in proportion to the rate at which the boiler is called upon to satisfy the load demand and staying at the preferred process variable (PV) set-point.
2. Discussion of the Prior Art
Various systems are disclosed in the prior art that modulate the firing rate of multiple boilers in a coordinated fashion, so that they jointly meet the load demands of a heating system or other industrial process. Several examples that require modulation of the firing rate of multiple boilers include: a heating system that requires steam to maintain the temperature in a building, kitchen steam absorption chillers, industrial processors, or any other consumer of steam or hot water that demands steam or hot water at a preferred level, e.g. retorts and cookers. There is a need to control the firing rates of multiple boilers to efficiently meet the load (output) demands. For example, the Bartels U.S. Pat. No. 4,513,910, operating under hysteresis, discloses a boiler operating system in which the boiler firing rate is a function of the boiler pressure, whereby the boiler operating system provides for the adjustment of the fire rate as the load demand on the boiler increases or decreases the boiler pressure. The control mechanism described in the patent automatically switches to a normal high fire and modulating mode if the boiler demand cannot be met at the low fire operating point.
The Shprecher et al. U.S. Pat. No. 5,042,431 describes a microprocessor based sequencer for a multiple boiler heating system. Each boiler or stage in a multiple boiler system is provided with an adjustable firing level of modulation at which the boiler is turned on, and an adjustable threshold level of modulation (the Mod. Pt.), below which the next stage is disabled from being turned on. A control device for the system continuously compares temperature in the medium being heated to a set-point temperature for the system and determines the total change in the output level which would be required to produce a specified temperature within a predetermined time. The microprocessor then adjusts the firing rate to meet this demand. This demand is spread equally among successive stages.
The Christiansen U.S. Pat. No. 5,172,654, assigned to applicant's assignee, describes a microprocessor-based boiler controller that base loads individual boilers at their most efficient firing rate. For example, in a base load mode of operating three boilers, on original start-up, the first boiler carries the load until its firing rate reaches its programmable "Add boiler load set-point" (which may be, for example, about 45 percent). At this time, the second boiler fires and is held at "low fire" for a fixed time sufficient to alleviate some damage due to thermal shock. The second boiler then follows the load in parallel with the first boiler until the second boiler reaches an "Effect Base Load set-point", at for example 25 percent.
Whenever possible, one or more boilers are allowed to operate at their preferred load at which their efficiency in combination is a maximum. Additional boilers are added to the system in like manner as the output demand increases. The boilers' firing rates are increased or decreased by a fixed percentage. By providing an automated base load feature, considerable fuel savings over the parallel mode of boiler operation can be realized. Further, by allowing intermittent warm up of the idle boilers, less repair and down time are experienced.
As those skilled in the art can appreciate, a certain amount of repairs and downtime from thermal shock is attributable to increasing the firing rate from low fire to high fire at a fixed rate or at a rate proportional to time. Also, the firing rate in these systems may overshoot the desired set-point or start up when the demand is low, thereby wasting fuel. Further, another frequent problem occurs when there is a large offset between the process variable (output) and the desired set-point (desired output). The prior art controls often increase the firing rate more than is required, and may even unnecessarily increase the firing rate on multiple boilers, thereby wasting fuel, and causing thermal shock to the tubes and refractory. Also, when more than one boiler is simultaneously increased to high fire, there is a rapid flux in the demand for water, often resulting in the water control sensors needlessly instituting a failure of the water level, and causing an unnecessary shutdown of the boiler.
Therefore, to further reduce the amount of wasted fuel, and the deleterious effects of thermal (repairs, downtime and premature wear on the boiler tubes), a control over the firing rate of the boilers as a function of the rate of change in output is needed in addition to normal proportional response. These and other disadvantages of the prior art are overcome by controlling the firing rate with proportional control of the firing rate but integrating the recovery rate into the control algorithm independently on each side of the PV set-point, thereby allowing the system to respond more slowly on one side of the set-point than on the other. By controlling the firing rate in this manner, the need for a second boiler, in some circumstances, may be eliminated. Further, the need for two boilers to run at high fire for several minutes may be avoided, which also decreases the amount of fuel spent. More importantly, thermal shock is prevented by increasing the firing rate, taking into account the rate of recovery of the process variable, rather than moving to high fire whenever the process variable falls below the set-point by a given offset.